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SrFe 12 O 19 is widely used due to its low cost, stability and corrosion resistance. In this study, Sm-doped porous SrFe 12 O 19 powder with high coercivity was synthesized using KCl- assisted spray pyrolysis. The Sm doping, microstructure and magnetic properties were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The results showed that when Sm-doping amount was 0.2, the ferrite powder prepared by KCl-assisted spray pyrolysis had high coercivity of 7.286 kOe and saturation magnetization of 56.422 emu/g. The coercivity of the Sm-doped samples increased by 39.23 % compared with that of the undoped samples. The significant increase in coercivity was associated with the improvement of super-exchange interaction between Fe3+ particles caused by Sm doping, and the size decrease of the hexagonal plate Sr-ferrite particle caused by KCl action. The influencing mechanism of coercivity was discussed in detail based on the analysis of crystal structure and microstructure observation. [ABSTRACT FROM AUTHOR]

Highlights • Ba-ferrite nanoparticles with good properties are synthesized by optimized process. • These nanoparticles show high coercivity and good thermal stability. • Excessive Ba ions is necessary for the synthesis of single phase particles. • The particle sizes are well controlled in the single domain size range. Abstract Nano-powders with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe 12 O 19) have been synthesized via a co-precipitation/calcination technique. The phase composition, morphology and magnetic/thermal properties of the products were systematically studied. XRD patterns reveal that a long co-precipitation reaction time (5 h) and high calcination temperature (1100 °C) are beneficial for the formation of BaFe 12 O 19 phase and decreasing the tendency to agglomeration. SEM micrographs show that the products show a hexagonal flake-like particle shape and the size are well controlled and maintained at single-domain particle size range area (<460 nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. The products with j H c of 5934 Oe, temperature coefficient of remanence (α Br) of −0.176% K−1 and temperature coefficient of coercivity (β jHc) of 0.0427% K−1 were obtained when co-precipitated for 5 h and calcined at 900 °C for 2 h. A high saturation magnetization of 66.9 emu/g was obtained when co-precipitated for 5 h and calcined at 1100 °C for 2 h, approaching the theoretical saturation magnetization (72 emu/g). [ABSTRACT FROM AUTHOR]

Abstract: The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker–Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially. [Copyright &y& Elsevier]

Abstract: Energy absorption produced by non-interacting single-domain ferromagnetic particles suspended in a liquid matrix is studied. The particles are assumed to be rigid dipoles so that all the dissipation in the system is due to the viscous flow induced in the matrix by orientational motion of the particles in response to a linearly polarized uniform AC field. The kinetic approach employed is valid for arbitrary values of the field frequency and amplitude. Comparison of the results obtained against those of the conventional phenomenological description reveals that the amplitude–frequency dependencies delivered by the latter are limited to weak field regimes. [Copyright &y& Elsevier]